1. Field of the Invention
The present invention relates to precision transfer equipment, more specifically, equipment that transfers a work table or a column of measuring equipment and a machine tool with a high precision for positioning.
2. Description of Related Art
Precision transfer equipment requiring a high motion accuracy (e.g., a precision positioning system, precision measuring equipment and a machine tool) has been designed to directly reflect a profile accuracy of a precisely finished guide rail to motion accuracy of an object to be moved.
Such precision transfer equipment is required to transmit a thrusting force only in a predetermined feed direction without transmitting vibration and the like from a driving source for transmitting a driving force by a drive mechanism to a movable body such as a table (an object to be moved). One of such transmission mechanisms exhibiting excellent performance is a static-pressure joint that supplies fluid between a pair of highly precise planes to form a static-pressure clearance.
Especially, a supplying-discharging static-pressure bearing that supplies fluid in an outer periphery of the static-pressure clearance and discharges the fluid in the middle, or a vacuum-balanced static-pressure air bearing can provide a strong rigidity in a compression direction and a tensile direction. Thus, there has been proposed to use a drive mechanism using the static-pressure bearing having such a high rigidity to increase geometric accuracy of motion of a high precision linear table (see Patent Literature 1: JP-A-2006-205292).
According to Patent Literature 1, only the thrusting force is transmitted in non-contact from the rod (a driving body) to the table (the movable body) via a static-pressure air bearing, so that transmission of vibration from the driving source is avoidable and a high motion accuracy of the table is achievable. Further, by placing a thrusting axis from the rod near the centroid of the table, the motion accuracy of the table is maintainable even in driving for acceleration or deceleration.
The static-pressure joint disclosed in Patent Literature 1 further includes a thrust plate that is supported near the rod by a gimbal mechanism absorbing a motion error in a yawing direction and is supported near the table by a similar gimbal mechanism absorbing a motion error in a pitching direction. Through the joint, forces other than the force in the moving direction of the table are not transmitted.
In order for precision transfer equipment to enhance accuracy, it is crucial to accurately detect and control a position of the movable body in a feed direction, in addition to a high geometric accuracy of motion of the drive mechanism and the movable body.
In order to provide such a function, there has been known a technique for detecting a position of a movable body in a feed direction by setting a laser interferometer in the precision transfer equipment (see Patent Literature 2: JP-A-2000-55617).
According to Patent Literature 2, a slider driving mechanism using a feed screw axis is provided under a table and a position detecting mechanism using a laser interferometer is provided on a side of the table. In this arrangement, a laser path extending from a light source of the laser interferometer to an end of the table is surrounded by a stretchable tube with a bellows, in which an inner pressure of the tube is reduced to eliminate an influence from air turbulence in the laser path, thereby achieving further high precision.
A simple combination of the precision transfer equipment disclosed in Patent Literature 1 and the positioning detecting mechanism disclosed in Patent Literature 2 entails the following problems.
For a precise positioning by the precision transfer equipment of Patent Literature 1, it is desirable that an object to be precisely positioned is placed on a driving axis as well as on a laser optical axis (a measurement axis) of the laser interferometer. However, it is difficult to coaxially arrange the laser path of the laser interferometer of Patent Literature 2 and the driving axis by the rod and the static-pressure air bearing of Patent Literature 1, where so-called offset is inevitable.
For this reason, even if a high precision is achieved by the precision transfer equipment of Patent Literature 1, a highly precise positioning detecting mechanism by the laser interferometer as disclosed in Patent Literature 2 cannot be combined with the precision transfer equipment of Patent Literature 1, so that a highly precise movement and a highly precise positioning may not be sufficiently achieved in practical use.
Moreover, Patent Literature 2 discloses that the bellows is used for reducing the pressure in the laser path in order to eliminate an influence of a medium in the laser path, in which the end of the bellows needs to be hermetically connected to the table. Accordingly, even if deviation of the thrusting axis is resolved by the gimbal mechanism as disclosed in Patent Literature 1, a high precision may be impaired by being affected by the bellows.